The practice of dentistry includes the preparation and application in the oral environment of artificial tooth structures and restorations. Such restorations include, for example, veneers, inlays, onlays, crowns and bridges. Historically, the dental profession has used several different types of materials for aesthetic restorative purposes. Unfilled acrylic resins, silicate cements and direct fill resins have all been used. Dental composite materials, which are particularly preferred for the purpose of aesthetic restorations, include an inorganic filler component such as glass and an organic matrix component such as a polymerizable monomer. Such composites typically comprise an acrylic or methyacrylic based system in which a silica or silicate glass is bonded to the resin matrix or to a coupling agent which is bonded to both.
Dental restorations are typically prepared by forming a restorative structure from an appropriate resin or composite material in the oral cavity or by placing the appropriate resin or composite material on a laboratory working model die made from an intra-oral impression replica or alloy substrate and curing the restorative material. The composite materials currently in use for such restorations involve the conversion of monomers and/or oligomers into a polymerization matrix by chemical or photochemical initiation to form free radicals and thereby effect polymerization.
Chemical initiation is generally effected by admixing substantially equal amounts of two paste or powder/liquid systems, one containing an initiator--usually peroxide or other free radical generating material and the other containing an organic amine accelerator -which react to produce free radicals and thereby initiate the polymerization reaction.
The light curing systems are based upon the discovery that certain chemical compounds will initiate polymerization of component monomeric units when exposed to light waves, in either the ultra violet or visible light region of the spectrum. U.S. Pat. No. 4,411,625, for example, describes composite dental restorative compositions which are curable by the action of light having visible and/or UV components. Photochemical initiation provides the ultimate flexibility in placement and working with the restoration since the monomers and/or oligomers are substantially unreactive until exposed to an appropriate light source which initiates polymerization.
Heat and pressure cure systems are also used in the conversion of monomers and/or oligomers into a polymeric matrix for dental restorations.
Although the foregoing modalities of cure have been extensively used and are currently in widespread practice in the dental profession, it has been found that each exhibits certain disadvantages. For example, visible light curing techniques frequently result in incomplete cure of the restorative dental resin or composite and produce a restoration having less than optimum mechanical and physical properties. Dental restorations formed by heat and pressure curing of composite resins also often exhibit inferior properties. In addition, dental laboratories which prepare dental restorative materials by heat and pressure cure require high pressure polymerization pots or containers, which are dangerous to use. Chemical initiation systems are highly inflexible as a result of the occurrence of rapid polymerization upon mixture of the two components and void inclusion. As a result, there has been a perceived need in the dental profession for a curing methodology which would result in an aesthetic dental restoration exhibiting superior properties, which is also safe and easy to use.
Accordingly, it is an object of the present invention to provide a novel method of preparing dental restorative materials which overcomes the deficiencies of the prior art systems.
It is another object of the present invention to provide a method for preparing polymerized dental restorative materials exhibiting greater fracture toughness and flexural strength and lower polymerization shrinkage than the currently available heat/pressure cure materials and the visible light cured materials in order to nullify post-operative sensitivity, improve marginal integrity and fracture and wear resistance.
A still further object of the invention is to provide a dental restoration prepared in accordance with the novel methodology.